Add first/last value assertions

[peter-tackage]

To make it simpler to verify Observables which emit multiple values, there should be an assertion which can operate on the first and last emissions.

// For first values
assertThat(ts).hasReceivedFirstValue(value);
assertThat(ts).hasReceivedFirstValueWhich().hasLength(5);
assertThat(ts).hasReceivedFirstValuesWhich().isEmpty(); // List assertion

// Similarly, For last values
assertThat(ts).hasReceivedLastValue(value);
assertThat(ts).hasReceivedLastValueWhich().hasLength(5);
assertThat(ts).hasReceivedLastValuesWhich().isEmpty(); // List assertion

[peter-tackage]

assertThat(ts).hasReceivedFirstValue(value); done in 14bc627

assertThat(ts).hasReceivedLastValue(value) done in 3213161

assertThat(ts).hasReceivedFirstValueWhich() and assertThat(ts).hasReceivedLastValueWhich() done in 59385d8

[peter-tackage]

Naming needs some thought for assertion of an individual List to avoid confusion with the other assertions which are asserting again all the emissions (as a List as received by TestSubscriber)

[peter-tackage]

Won't do the List implementation yet. I don't support it for single general elements currently.